An admission control function can be implemented when establishing or opening a service session. A service session allows a data server to provide a client entity with data that may represent a digital content for example sounds or images. The service session may be accepted or rejected.
This admission control function makes it possible to protect the quality of service of the transmission of the data of already established sessions by rejecting, if necessary, new sessions.
Thus, the admission control function must ensure, for any new request for session opening on the part of a client entity, that the resources necessary for this new session are available in the network in order to accept the request in a time acceptable to the user of the client entity. This time depends on the service concerned.
This admission control function is relatively simple to implement in communication networks operating in connected mode by virtue of an exchange of signaling preceding any session opening and making it possible to verify whether the necessary resources are available and to reserve them. This admission control function is termed explicit, since the signaling request clearly indicates the amount of resources to be reserved, for example, the bitrate. However, an under-usage of resources is noted for networks of this type.
So-called implicit admission control functions also exist, which do not require a signaling exchange indicating the amount of resources to be reserved.
For this purpose, the patent application published under the number WO2011/001096 proposes a technique for controlling the opening of communications sessions between a server and client terminals linked by a packet communication network, in which, on receipt of a congestion information item regarding a first communication session established between a first client terminal and the server, a control device inhibits the opening of a second communication session with a second client terminal. A congestion information item indicates the existence of congestion. This entails for example the presence of the information item CE “Congestion Experienced” conveyed by the ECN field of the header of an IP packet.
Provision is thus made to detect the presence of an information field optionally containing a congestion information item relating to congestion affecting the first session and as a function of the value of the detected information field, to control (that is to say accept or refuse) the opening of the second communication session with the second client terminal. However, this procedure only makes it possible not to amplify an already identified congestion phenomenon by not admitting any new service sessions. It does not make it possible to obtain all the effects of an explicit admission control, such as the guarantee for the service sessions admitted of a quality of service in accordance with that required.